Lately I have been buying my RO/DI water from the LFS. At home I have a GE smartwater RO drinking water system that I was originally using as my source for FW. I stopped using it because it seems to contain chloramine (water tests shows small levels of ammonia). My question is can I simply add a DI stage to this unit and use the water it produces for my reef? Is there any limitation or shortcoming of this system versus one of the RO/DI units "designed" for reef use? Or do I really need to spend the cash on a RO/DI unit?

you can definitely add a DI stage to it, as long as all of the other filters and the membrane are in good shape you should be fine, but only water tests can tell you for sure. I would say go for it and while you are at it pick up a TDS meter if you don't already have one, to test your existing filters.

You should not be seeing ammonia after the GE unit, I would bet its time to change all the filters in it. You should ideally get a TDS meter and see what the water coming out of the unit reads. If it reads much higher than 10ppm any DI stage you add will be exhausted quickly. Since DI resin can get expensive to replace, you need to make sure the rest of the system is working optimally to use the resin as slow as possible.

Thanks for the responses, I replaced ALL filters and the membrane on the GE RO unit recently. It still tests for ammonia after replacing all filters. I think I read somewhere that RO units cannot effectively remove chloramines, only DI's. I ordered a TDS meter last week to test my LFS's RO/DI and will use that to see what the TDS is after the RO unit and report back here.

"They" say that chlorine is really bad for RO membranes, so if you had a problem with your prefilters at some point, your RO membrane may have been compromised. I had this happen to me several years ago and didn't realize it until the membrane pretty much let loose and I was getting really high pressure water out of the RO unit. Fortunately I wasn't doing fish at the time, so no aquatic life was harmed. :)

Anyway, if you haven't already checked/changed your membrane, you might look into that as well.

FWIW, I picked up the Drs. Foster and Smith DI add-on kit a while back. It was priced competitively and fit nicely into my system.

Just got my TDS meter in today. My tap water is at 242ppm, and the RO water from the GE unit is 18ppm. For references sake my LFS's RODI is at 3ppm. All filters as well as the RO membrane on the GE unit are brand new. Is this performance acceptable enough to add a DI stage? I don't want to burn through too much DI resin, but dont know whats a good number after the RO stage.

Mine is 15 after the RO stage and I've had the same DI resin for a year now. Still putting out 0PPM water. When you get the DI cartridge, mount it verticaly, not horazontaly, so the water HAS to go through the DI resin instead of being able to flow over it.

Thanks for the response Phurst. I see a DI stage in the near future for me :-). No more trips to the LFS for RODI.

Just to add my two cents,

I have a spectrapure DI unit attatched to the RO drinking water system in my house. It has worked very well. TDS after RO is 11 and after the DI is now 1 (was zero for ~ 8 months).